(1) Field of the Invention
The present invention relates to an apparatus and method for measuring a running direction of a vehicle using a geomagnetic direction sensor. The present invention, more particularly, relates to the apparatus and method for automatically correcting the measured running direction of the vehicle including an error caused by a magnetization on a vehicle body.
(2) Description of the Background Art
One of previously proposed apparata for measuring running directions of vehicles is exemplified by a Japanese Patent Application First Publication (Non-examined) Shows 59-100812 published on Jun. 11, 1984.
In the above-described Japanese Patent Application First Publication, a geomagnetic direction sensor, in which a pair of windings are intersected perpendicularly to each other on a circular core in a horizontal posture, is used for measuring the running direction of the vehicle. Detected voltages (output values) corresponding to the geomagnetic directional components are derived respectively according to interlinked geomagnetism to the respective windings.
When the vehicle is made one turn under the uniform geomagnetism, a circle (output circle derived from the geomagnetic direction sensor) is described on a coordinate plane prepared from an X - Y coordinate indicated by detected voltages of these core windings. Hence, a direction from a center of the output circle to the coordinates indicated by the detected voltages of both windings is derived as the forward direction of the vehicle during a normal running of the vehicle.
In the above-described direction measuring apparatus, center coordinates of the output circle is moved when the vehicle body is magnetized so that an error is generated in the detection result of the running direction. To correct the erroneous detection result, the vehicle is made one turn and during one turn of the vehicle the output values of the geomagnetic direction sensor are appropriately sampled each at a predetermined sampling period.
Then, upon completion of the one turn, the sampled output values are averaged to correct the error in the detected running direction of the vehicle.
On the other hand, another of the previously proposed vehicle running direction measuring apparata is exemplified by a Japanese Patent Application First Publication (Non-examined) No. Shows 63-48415 published on Mar. 1, 1988.
In the latter Japanese Patent Application First Publication, a method of correcting the erroneous detected output values due to the magnetization of the vehicle body is such that a plurality of phantom X axis and phantom Y axis are drawn in parallel to the X axis and Y axis on the VX-VY plane of the output of the direction measuring apparatus. If the outputs of the direction measuring apparatus are placed on two points on the phantom X axis and on two points on the phantom Y axis, the correction for the erroneous detected voltages due to the magnetization on the vehicle body is carried out.
However, the above-described previously proposed direction measuring apparata have the following drawbacks.
Since, in the former direction measuring apparatus, a small amount of sampled output values are used to process the average calculation even when a magnetism environment is not preferable during the correction of the magnetization, it is not possible to derive the position of the center coordinates of the output circle with high accuracy and the vehicle driver is required to make the vehicle one turn (360.degree.).
Since, in the latter direction measuring apparatus, two points of intersections between the two axis (X axis, Y axis) and directional circle are needed, the directional data of at least 180.degree. or greater is required to measure the intersections.
However, since 180.degree. turning of the vehicle is rare immediately after the vehicle is actually magnetized, the output of the direction measuring apparatus until the directional data of more than 180.degree. are collected is deviated from the actual direction of the vehicle.